With the great increase of global energy consumption, the development of highly efficient and low-cost renewable energy resources becomes extremely important and necessary, in which the most feasible technology is to directly convert solar energy into electronic power by solar cells.[1,2] As an attractive alternative to conventional silicon-based solar cells for low-cost clean energy, dye sensitized solar cells (DSSCs) have received considerable interests due to its low production cost, simple fabrication processes, environmental friendliness, and relatively high photoelectric conversion efficiency.[3-6] For DSSCs, achieving high energy conversion efficiency is one of the most important keys to the future commercialization in the huge electricity generation market.[1,7] In the past few decades, the development of DSSCs can be regarded as a process of continuously improving efficiency since nano-crystalline DSSCs were reported by Grätzel et al. in 1991.[1-8] The energy conversion efficiencies over 12% has been achieved for DSSCs in the lab size devices,[5] however, it is still far lower than the current multicrystalline silicon solar cells (˜20%),[9] and there is a large space for the improvement of DSSCs performance.[10] Therefore, developing new materials and structure with low-cost fabrication technique for DSSCs, and understanding the intrinsical mechanisms of photo-electron conversion are highly desired.
In general, a typical DSSC comprises a dye-sensitized TiO2 nanocrystalline porous film coating on a transparent conductive oxide (TCO) glass substrate as the photoanode, a liquid redox electrolyte containing an I−/I3− redox couple, and a platinum (Pt) catalyst as the counter electrode (CE).[3] The photoanode containing porous TiO2 nanoparticles (NPs), one of the most important components in DSSCs, is responsible for adsorbing dye molecules, transferring the photogenerated electrons from the dye to TiO2 and to the conductive substrate, and providing a diffusion path for redox ions, which importantly influences the charge recombination, electron collection and transportation rate, and the light absorption.[11,12] Therefore, the characteristics of the porous TiO2 NPs, especially the morphology and size, interparticle connectivity, pore structure, and electric structure, are greatly vital in determining the final photovoltaic performance of DSSCs.[13] It has been demonstrated that the wide band gap of TiO2 (3.2 eV) and high recombination rate of photogenerated hole-electron pairs are main limitation factors for its performance improvement and widespread application in industry.[14] In order to address these problems, several strategies, such as dye sensitizing, heterostructure, ion doping, etc., have been developed.[14-17] Among them, the ion doping is believed to be the most economical and facile method to optimize the performance of TiO2 NPs by simply modifying its structure (morphology, size, electronic structure), which has been widely used in the photocatalysis, biological engineering and gas sensors fields,[18-21] but there are relatively few studies applied in DSSCs.[15,22] More recently, in order to further improve the efficiency of DSSCs, several efforts has been tried to modify nanostructured TiO2 NPs by metal ions doping such as Er3+, Yb3+, Mg2+, Zn2+, Mn2+, Co2+, Sn4+, and Nb5+, aiming to enhance electron transportation and suppress charge recombination.[12,22-26] Of these doping elements, the Nb stands out to show great potential in improving DSSCs performance due to the synergistic advantages of superior electrical conductivity, similar atom radii with Ti, high valence favorably enhancing free carriers, and excellent ability in stabilizing the phase structure and tailoring the optical properties. For example, Lü et al.[26] and Nikolay et al.[22] have synthesized Nb-doped TiO2 particles by a hydrothermal method and used them in high efficiency DSSCs. However, an important limitation for the practical application of the Nb-doped TiO2 NPs applied in DSSCs is that the starting materials either use expensive niobium ethoxide or the preparation method adopts the hydrothermal technique which includes multi steps and long time,[22,26,27] thereby leading to high preparation cost and low production efficiency. For example, Nb-doped TiO2 was prepared by a sol-gel method followed by a hydrothermal treatment in Lü et al; the original sources of TiO2 and Nb in Nikolay et al. were from niobium powder and tetrabutyl titanate, which were added into hydrogen peroxide and ammonia (5:1 v/v) to obtain the precursor and followed a series of procedure. Both of these conventional methods are more complicated and in higher cost to synthesize the TiO2 and precursor.
Therefore, a facile one-pot synthesis as an economical, simple, and high yield approach for preparing Nb5+-doped TiO2 nanoparticles for use in highly efficient DSSCs with a high energy conversion efficiency is highly demanded.